Conventionally switching power supplies are widely used, each serving as a power source supply installed in office automation equipment such as a personal computer. In such a power supply, switching is performed on direct-current input voltage by a switching element, alternating voltage generated by the switching operation via a transformer is rectified and smoothed to output direct-current input voltage, and when the direct-current output voltage is fed to a load, the switching operation of the switching element is controlled by a control circuit according to a change in direct-current output voltage, so that direct output voltage is stabilized.
The above conventional switching power supply (e.g., JP10-304658A) will be described below with reference to the accompanying drawings.
FIG. 7 is a circuit block diagram showing a structural example of a switching control section in the conventional switching power supply. As shown in FIG. 7, a switching control section 62 has a switching element 1 such as a power MOSFET and a control circuit for performing the switching control of the switching element 1 that are integrated on the same semiconductor substrate. The switching control section 62 is constituted of a semiconductor device (hereinafter, the switching control section will be indicated as a semiconductor device) having nine terminals of an input terminal 53 and an output terminal 54 of the switching element 1, a terminal 55 for detecting overvoltage protection and starting voltage, a power supply terminal 56 of the control circuit, a terminal 57 for detecting remote on/off and overload/overcurrent protection, a terminal 58 for detecting a heavy load, a control terminal 59 for inputting a control signal, a terminal 60 for detecting a bias winding voltage of a transformer, and a terminal 61 for connecting a capacitor which determines a switching frequency of the switching element 1.
In the semiconductor device 62, a regulator 6 connects the input terminal 53 of the switching element 1, the terminal 55 for detecting starting voltage, and the power supply terminal 56 of the control circuit. When the input terminal 53 of the switching element 1 has a voltage of a given value or higher, the internal circuit current of the semiconductor device 62 is fed and control is performed by a regulator comparator 8 so that the voltage of the power supply terminal 56 for the semiconductor device 62 has the given value. The output of a start/stop comparator 7 is inputted to an AND circuit 18, the output signal of the AND circuit 18 is inputted to a NAND circuit 51, and the resonance (switching operation) and stop of the switching element 1 are controlled according to a voltage of the terminal 55.
Reference numeral 9 denotes an overvoltage protection circuit. Voltage is detected from the bias winding of the transformer via a rectifier, so that when output voltage from the secondary side of the transformer increases too high, a NAND circuit 20 outputs a signal to the set terminal (S) of an RS flip-flop 21 and the operation of the switching element 1 is stopped in a latch mode. A return is made to the operation from overvoltage protection when a restart trigger signal 22 is outputted to the reset terminal (R) of the RS flip-flop 21.
Reference numeral 10 denotes an overheat protection circuit. When the chip temperature of the semiconductor device 62 is equal to or higher than a set value, the NAND circuit 20 outputs a signal to the set terminal (S) of the RS flip-flop 21 to stop the operation of the switching element 1. A return is made to the operation from overheat protection when the restart trigger signal 22 is outputted to the reset terminal (R) of the RS flip-flop 21.
Reference numeral 15 denotes a clamping circuit which performs control so that the terminal 57 has a potential of a constant value.
Reference numeral 17 denotes a remote on/off detection circuit. The circuit controls the potential of the terminal 57 outside the semiconductor device 62, so that the switching element 1 is forcibly stopped (remote off) or is returned to an operating state (remote on).
Since a resistor is connected to the terminal 58 from the outside of the semiconductor device 62, a constant voltage is set by a constant-current source 23. Further, the voltage is inputted to a heavy load detection circuit 24 and is set as a heavy load level.
Reference numeral 26 denotes a clamping circuit which is connected to the control terminal 59. Since a photocoupler is connected to the control terminal 59 from the outside of the semiconductor device 62, a constant potential is set on the terminal 59. Reference numeral 27 denotes an IV converter which internally converts current fed from the control terminal 59 into voltage.
A high-side clamp 30 and a low-side clamp 31 are connected to the terminal 60 for detecting the bias winding voltage of the transformer and limit voltage inputted to the inside of the semiconductor device 62. Further, a transformer reset detection circuit 32 is connected to the terminal 60. The timing of the turn-on signal of the switching element 1 is determined by a one-shot pulse generation circuit 33.
Reference numeral 19 denotes a start pulse generation circuit. Output is generated by the output signal of the start/stop comparator 7, that is, a start signal, and the output signal of the remote on/off detection circuit 17, that is, the output signal of the AND circuit 18 that is a remote signal. The output is inputted to the set terminal (S) of an RS flip-flop 43 through an OR circuit 34 and an AND circuit 68, and the output Q of the RS flip-flop 43 is inputted to the NAND circuit 51.
A capacitor is connected to the terminal 61 from the outside of the semiconductor device 62 and a potential of the terminal 61 is fixed at a certain potential by a high-side forcing clamp 38 before startup. As to the terminal 61 after startup, the output signal Q of the RS flip-flop 43 is set at H and the switching element 1 is turned on via the AND circuit 68 by a start pulse signal, or a one-shot pulse signal during a normal operation. At the same time, the potential of the terminal 61 is reduced from a high-side clamp potential.
Further, when a switch 46 is turned on by the output signal of the RS flip-flop 43, electrical charge accumulated on the terminal 61 is discharged by the constant-current source 47, and it is detected by a comparator 40 that the potential of the terminal 61 is lower than the voltage value internally converted by the IV converter 27, an N-type MOSFET 42 is turned on via an OR circuit 41 and the charge on the terminal 61 is forcibly discharged.
When it is detected by a comparator 35 that the potential of the terminal 61 is at a given potential (band gap voltage) or lower, an H signal is inputted to the reset terminal (R) of the RS flip-flop 43 via an OR circuit 36 and the switching element 1 is turned off. At this point of time, a switch 45 is turned on and charging is started on the capacitor externally connected to the terminal 61.
When it is detected by a comparator 37 that the potential of the terminal 61 increases higher than a given voltage (at a heavy load: about 2.5 V) or the voltage (at a light load) obtained by voltage conversion of the IV converter 27, a P-type MOSFET 39 is turned on by the output signal of the comparator 37, the potential of the terminal 61 is placed into a high-side forced clamping state, and the terminal 61 is fixed at a certain voltage. Thereafter, when the output signal of the one-shot pulse generation circuit 33 is inputted to the OR circuit 34, the switching element 1 is turned on.
In this way, the on/off period of the switching element 1 is determined by the output voltage of the IV converter 27, the output voltage being internally subjected to voltage conversion by means of current fed from the control terminal 59, and the output signal of the one-shot pulse generation circuit 33 for generating a one-shot pulse from the output of the transformer reset detection circuit 32 which detects the voltage of the terminal 61 and the bias winding voltage of the transformer to determine the timing of turning on the switching element 1. Further, an operating frequency of the switching element is determined according to a capacity value of the capacitor externally attached to the terminal 61.
Moreover, a capacitor is connected from the outside of the semiconductor device 62 to the terminal 57 for detecting remote on/off. In the case of a heavy load, a P-type MOSFET 12 is turned on by the output Timer of an AND circuit 29, and current is charged, by a constant-current source 11, to the capacitor externally attached to the terminal 57. When an overcurrent protection circuit including an overcurrent protection detecting comparator 48 is operated, a P-type MOSFET 14 is turned on by the output OC of an AND circuit 50 and current is similarly charged, by a constant-current source 13, to the capacitor externally attached to the terminal 57.
When an overloading state or a state of overcurrent protection continues, the capacitor connected to the terminal 57 is increased in potential, an overload/overcurrent abnormality protection circuit 16 causes the NAND circuit 20 to output a signal to the set terminal (S) of the RS flip-flop 21, so that the operation of the switching element 1 is stopped. A return is made from overload protection and overcurrent protection is made when the restart trigger signal 22 is outputted.
FIG. 8 is a circuit diagram showing an example of the clamping circuit 26, the IV converter 27, and a soft start generation circuit 25 that are connected to the control terminal 59. In FIG. 8, the clamping circuit 26 is constituted of a constant-current source 209, a resistor 211, an NPN type bipolar transistor 210, and N-type MOSFETs 212 and 213. The control terminal 59 is set at a given potential. The IV converter 27 is constituted of a constant voltage source 201, an NPN type bipolar transistor 202, a resistor 203, and an N-type MOSFET 206.
A constant-current source 215 limits current when the terminal 59 shorts out with the ground. A constant-current source 218 is provided to make negligible dark current on a photocoupler which is externally attached to the control terminal 59.
The soft start generation circuit 25 is constituted of a P-type MOSFET 219, an N-type MOSFET 220, a resistor 221, a capacitor 222, and a start signal.
Regarding a part surrounding the IV converter 27 configured thus, explanation will be made on operations which are simply divided for a heavy load and a light load. Typical soft start is used and thus the explanation thereof is omitted.
After startup, since the start signal is at L level, an N-type MOSFET 220 is turned off and a P-type MOSFET 219 is turned on. First, in the case of a heavy load, current fed from the control terminal 59 is extremely low, current fed to a P-type MOSFET 216 is reduced, and current fed by a mirror circuit to a P-type MOSFET 217 is also reduced. Hence, current fed to the mirror circuit is reduced, the mirror circuit being constituted of an N-type MOSFET 207 and an N-type MOSFET 208, so that current fed by a constant-current source 204 is mainly applied to an N-type MOSFET 205.
Therefore, a large amount of current is applied to the N-type MOSFET 206 by the mirror circuit. Assuming that the current value is I, the constant voltage source 201 has a voltage value of V, the NPN type bipolar transistor 202 has a VF value of VF, and the resistor 203 has a resistance of R, the IV converter 27 has an output value VFB expressed by the equation below.VFB=V−VF−R×I  (Equation 1)In this equation, VFB has a small value.
However, in the case of a light load, a large amount of current is fed from the control terminal 59 and thus the current I applied to the N-type MOSFET 206 finally has a small value. Therefore, in the case of a light load, VFB expressed by (Equation 1) is changed to a large value.
FIG. 9 is a circuit block diagram showing a structural example of a switching power supply constituted of the conventional switching control section (as a semiconductor device) 62 shown in FIG. 7. In the switching power supply, a commercial alternating-current power supply is rectified by a rectifier 101 such as a diode bridge and is smoothed by an input capacitor 102, so that a direct-current voltage VIN is obtained and is fed to a transformer 103 for converting power.
The transformer 103 has a primary winding 103a, a secondary winding 103b, and a tertiary winding (bias winding) 103c. The direct-current voltage VIN is fed to the primary winding 103a. 
The direct-current voltage fed to the primary winding 103a of the transformer 103 is switched by the switching element 1 of the semiconductor device 62. Then, alternating current is drawn to the secondary winding 103b of the transformer 103 by the switching operation of the switching element 1.
The alternating current drawn to the secondary winding 103b of the transformer 103 is rectified and smoothed by a diode 104 and a capacitor 105 that are connected to the secondary winding 103b, and the alternating current is applied as the direct-current power of the output voltage VO to a load 109.
For example, an output voltage detection circuit 106 constituted of an LED 107 and a Zener diode 108 is connected across the capacitor 105 and outputs a feedback signal for stabilizing the output voltage VO to a phototransistor 110 on the primary side, the phototransistor 110 being connected to the control terminal 59 of the semiconductor device 62.
Further, the tertiary winding (bias winding) 103c of the transformer 103 is connected to the terminal 55 for detecting starting voltage and overvoltage, via a diode 113 and the terminal 60 for detecting a bias winding voltage.
Capacitors 111 and 112 prevent the terminal 55 and the terminal 56 for the power supply voltage of the control circuit from rapidly decreasing, that is, the capacitors 111 and 112 stabilize the terminals. A capacitor 114 connected to the terminal 57 stops the switching element 1 in a latch mode in the event of overload and overcurrent.
Further, a resistor 115 for setting a heavy load level is connected to the terminal 58, and the capacitor connected to the terminal 61 determines a switching frequency of the switching element 1. A capacitor 117 connecting the terminals 53 and 54 for the input/output of the switching element 1 determines a period and magnitude of resonance with the transformer 103.
The following will discuss the operations of the switching control section and switching power supply configured thus.
When an alternating-current power supply is inputted from a commercial power supply to the rectifier 101, rectification and smoothing are performed by the rectifier 101 and the capacitor 102 and conversion is made to the direct-current voltage VIN. The direct-current voltage VIN is applied to the primary winding 103a of the transformer 103.
When the direct-current voltage VIN is equal to or higher than a given value, charging current is fed to the capacitors 111 and 112 via the regulator 6 in the semiconductor device 62, the voltage of the power supply terminal 56 in the semiconductor device 62 reaches a given level, and the internal circuit is started. When the voltage of the terminal 55 reaches the starting voltage set by the start/stop comparator 7, control is started on the switching operation of the switching element 1.
Before startup, the terminal 61 is fixed on a certain potential by the high-side forcing clamp 38. In response to a signal from the start/stop comparator 7, a start pulse is generated from the start pulse generation circuit 19 and the switching element 1 is turned on. At this point of time, the switch 46 is turned on, electrical charge on a capacitor 116 connected to the terminal 61 is discharged by the constant-current source 47, and the terminal 61 gradually decreases in potential. Since the direct-current output voltage VO on the secondary side is low upon startup, current is not fed to the Zener diode 108 of the output voltage detection circuit 106 and thus current is not fed to the phototransistor 110.
However, in the control terminal 59 of the semiconductor device 62, charging current is fed to the capacitor 222 from the soft start generation circuit 25 shown in FIG. 8. The voltage VFB having been subjected to IV conversion by the IV converter 27 has a high value according to (Equation 1). When the voltage of the terminal 61 becomes lower than the voltage VFB, the N-type MOSFET 42 is turned on by the output signal of the comparator 40 and the electrical charge of the capacitor 116 connected to the terminal 61 is forcibly discharged.
When it is detected by the comparator 35 that the voltage of the terminal 61 is equal to or lower than a certain potential (band gap voltage), an H signal is inputted to the reset terminal (R) of the RS flip-flop 43 and the switching element 1 is turned off. At this point of time, the switch 45 is turned on and the charging of constant current is started from a constant-current source 44 to the capacitor 116 connected to the terminal 61. When the comparator 37 detects a voltage higher than a certain voltage (at a heavy load: about 2.5 V) or the VFB voltage (at a light load), the P-type MOSFET 39 is turned on and the potential of the terminal 61 is fixed at a certain potential, which is internally set, by the high-side forcing clamp 38.
Thereafter, according to a resonating operation determined by the leakage inductance of the transformer 103 and the capacities of the capacitor 117 and the switching element 1, when the voltage of the tertiary winding (bias winding) 103c of the transformer 103 changes from positive to negative, that is, when the input terminal 53 of the switching element 1 decreases in voltage, the output of the one-shot pulse generation circuit 33 is inputted in a state of high level to the set terminal (S) of the RS flip-flop 43 via the OR circuit 34 and the AND circuit 68 by the transformer reset detection circuit 32, and the switching element 1 is turned on.
The above-described switching operation is repeated and the direct-current output voltage VO is increased. At a voltage or higher than the voltage set by the output voltage detection circuit 106, the LED 107 is brought into conduction and thus current is fed to the phototransistor 110. Then, current from the phototransistor 110, that is, current from the control terminal 59 of the semiconductor device 62 is fed and the on duty of the switching element 1 is changed to a proper state.
Namely, the switching operation of the switching element 1 is turned on by a one-shot pulse which is the output signal of the transformer reset detection circuit 32 and is outputted from the one-shot pulse generation circuit 33, and the on-duty of the switching element 1 is determined by current fed from the control terminal 59.
That is, as shown in the time chart of FIG. 10, regarding current fed to the load 109, a light load (FIG. 10(b)) has a shorter period for feeding current to the switching element 1, a heavy load (FIG. 10(a)) has a longer period for feeding current to the switching element 1. In this way, the semiconductor device 62 performs control so that the on duty of the switching element 1 is changed according to a current fed to the load 109 of the switching power supply.
Further, the timing of turning on the switching element 1 is set so that output is performed when the switching element 1 has the lowest input voltage during the resonating operation, so that switching loss hardly occurs when the switching element 1 is turned on. That is, a partial resonating operation is performed so that switching loss is negligible when the switching element 1 is turned on.
With this operation, it is possible to achieve high efficiency or low noise in a normal operation.
In the conventional switching power supply, although current fed to the switching element is reduced at a low load in a standby state and so on, current has to be fed by the switching operation of the switching element via the transformer to the internal circuit of the switching control circuit of the switching control section, which is constituted of the semiconductor device, and current fed to the switching element cannot be set at 0. Thus, a certain amount of current is applied even at no load.
Therefore, even at no load, a loss is generated by the switching operation of the switching element 1. A lighter load increases a loss on the switching element 1 and reduces efficiency on the power supply. Thus, it is not possible to meet the need for lower power consumption during the standby mode of the power supply.